Fix parsing of signature inp/out types in predecls (#10642)

# Description
Fixes https://github.com/nushell/nushell/issues/10605 (again).

The loop looking for `[` to determine signature position didn't stop
early enough, so it thought the second `[` denoting the inp/out types
marks the beginning of the signature.

# User-Facing Changes

# Tests + Formatting
adds a new `predecl_signature_multiple_inp_out_types` test

# After Submitting

crates/nu-parser/src/parse_keywords.rs

@@ -202,6 +202,7 @@ pub fn parse_def_predecl(working_set: &mut StateWorkingSet, spans: &[Span]) {
                 .starts_with(&[b'('])
         {
             signature_pos = Some(pos);
+            break;
         }
 
         pos += 1;